In recent years, water-absorbent resins have been widely used for various fields, including hygienic materials such as disposable diapers and sanitary napkins, agricultural and horticultural materials such as water retaining materials and soil conditioners, and industrial materials such as water-blocking agents and dew condensation prevention agents. Among these fields, the water-absorbent resins are most often used especially for hygienic materials such as disposable diapers and sanitary napkins.
As such water-absorbent resins, there are known, for example, hydrolysates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylic acid graft polymers, saponified products of vinyl acetate-acrylic acid ester copolymers, and crosslinked products of partially neutralized polymers of acrylic acid.
Incidentally, absorbent articles represented by disposable diapers, sanitary napkins and incontinence pads are composed of an absorbent material for absorbing and retaining a body liquid discharged from a human body, such as urine and menses, the material being disposed mainly at the central part, a liquid-permeable front sheet (top sheet) positioned on a side where the sheet comes into contact with a body and a liquid-impermeable rear sheet (back sheet) positioned on a side opposite from the side where the sheet comes into contact with the body. Usually, the absorbent material comprises hydrophilic fibers, such as pulp, and a water-absorbent resin.
The absorbent material temporally retains a liquid having entered through the front sheet with the hydrophilic fibers and then retains the liquid with the water-absorbent resin. Generally, to increase the liquid absorption amount of the absorbent material, it is effective to increase the amount of the water-absorbent resin and thereby relatively reduce the amount of the hydrophilic fibers. In the case of having increased the amount of the water-absorbent resin and thereby having reduced the amount of the hydrophilic fibers, however, there is a drawback that when the absorbent material has absorbed a liquid, the volume of the absorbent material increases due to swelling of the water-absorbent resin, causing decrease in intertwinement of the fibers themselves or of the water-absorbent resin with the fibers. Such decrease in intertwinement of the fibers themselves or of the water-absorbent resin with the fibers becomes a serious drawback in the case of using the water-absorbent material as a hygienic material because when compression or a shearing force is applied to the absorbent material, deformation such as cracking or uneven distribution occurs, and moreover inhibition of liquid absorption or re-wet of a liquid occurs.
As a method for preventing deformation of an absorbent material, there has heretofore been known a method of developing an adhesion force between thermally fused fibers or between pulp and thermally fused fibers by incorporating the thermally fused fibers into the pulp, and thereby improving the shape retention property of the absorbent material (see Patent Document 1). The method of Patent Document 1, however, has a problem that the liquid permeation rate and the absorption amount lower because the thermally fused fibers are hydrophobic though the shape retention property of an absorbent material is improved.
Against this background, it has been desired to develop a water-absorbent resin capable of improving the shape retention property of an absorbent material when being used for the absorbent material.